At present, PS (presensitized) plates using a positive working photosensitive agent comprising mainly a quinonediazide compound and a phenol resin and a negative working photosensitive agent comprising mainly an acrylic monomer or a prepolymer thereof are practically used as lithographic printing plates. However, due to their low sensitivity, a platemaking is carried out by contact exposure using a film original to which an image has previously been recorded.
On the other hand, in recent years, with the progress in the computer image processing and the data saving of large capacity as well as the data communication technique, an electronic editing system has been put into practical use where input of originals, correction, editing, layout and page-making are operated throughout by a computer, and the data are instantly available to output at a terminal plotter through a high-speed communication network and a communication satellite even at a far place. In particular, the electronic editing system is highly demanded in the newspaper printing field where quickness is always required. Also, at present, in the field where originals are saved in the form of films and printing plates are duplicated from the films according to need, the original will be saved as digital data in recording media of large capacity such as optical disks, with the progress of such recording media.
However, the direct platemaking in which a printing plate is prepared directly from the output of terminal plotters has not yet been practically used, and the output is made with silver salt photographic films even where the electronic editing system is operated, and practically a printing plate is prepared by contact exposure from the photographic film to a PS plate. One reason therefor is that it is difficult to develop a direct-type printing plate which having a sensitivity sufficiently high to prepare a printing plate with a light source of the output plotter (for example, a He-Ne laser or a semiconductor laser) within a practically acceptable period of time.
It is considered that an electrophotographic photosensitive material would be a photosensitive material having a high sensitivity capable of providing a direct-type printing plate. As a method for preparing a printing plate using the electrophotography, a method comprising forming a toner image and eluting a photoconductive layer in the non-image area has been known. Electrophotographic lithographic printing plates using such a method are described, for example, in JP-B-37-17162, JP-B-38-6961, JP-B-38-7758, JP-B-41-2426, JP-B-46-39405, JP-A-50-19509, JP-A-50-19510, JP-A-52-2437, JP-A-54-145538, JP-A-54-134632, JP-A-55-105254, JP-A-55-153948, JP-A-55-161250, JP-A-57-147656 and JP-A-57-161863. (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.)
In the above-described method, the non-image area of the electrophotographic photosensitive material must be removed with an eluting solution to expose a hydrophilic surface. However, the method has a defect in that the eluting solution often permeates into a photoconductive layer beneath the image area, i.e., the area where a toner image has been formed, and removal of the image area, i.e., a so-called "side-etching" occurs whereby highlight portions or fine lines which must be retained are removed and, hence, the image reproducibility is decreased.
To the above defect, a lithographic printing plate using a photoconductive compound and a photosensitive material in its photosensitive layer for the purpose of providing compatibility between the high-sensitivity of the electrophotographic system and good printing image reproducibility of the positive working photosensitive layer has been proposed, for example, in JP-A-57-90648, JP-A-58-150953, JP-A-58-162960 and JP-A-58-162961. The positive working photosensitive layer disclosed therein uses an o-quinonediazide compound as a photopolymer which is a photosensitive material. For this reason, the photoconductive compound functions as a filter to the photopolymer thereby reducing the sensitivity at the time of the whole surface exposure and, hence, there is a disadvantage in that the time for the whole surface exposure must be prolonged markedly. Also, the printing plate using a photoconductive compound such as a pigment is desired to have a high eluting latitude at the time of elution of the non-image area, but the above-described conventional technique is directed to a method for forming a printing image by difference in solubility caused by light-exposure, the difference in the solubility between the non-image area to be removed with the eluting solution and the image area to be retained is not necessarily high, and, therefore, the conventional technique is not satisfactory in the eluting latitude.
As described above, the conventional technique has problems in that productivity can not be improved since the exposing time at the time of the whole surface exposure is required to be prolonged or the eluting latitude is not necessarily satisfactory.